Because of their strength, heat resistance, and chemical resistance, polyester containers, films, sheets, and fibers are an integral component in numerous consumer products manufactured worldwide. In this regard, most commercial polyester used for polyester containers, films, and fibers is polyethylene terephthalate polyester.
Polyester resins, especially polyethylene terephthalate and its copolyesters, are also widely used to produce rigid packaging, such as two-liter soft drink containers. Polyester packages produced by stretch-blow molding possess outstanding strength and shatter resistance, and have excellent gas barrier and organoleptic properties as well. Consequently, such lightweight plastics have virtually replaced glass in packaging numerous consumer products (e.g., carbonated soft drinks, fruit juices, and peanut butter).
In conventional processes for making polyester resins, modified polyethylene terephthalate resin is polymerized in the melt phase to an intrinsic viscosity of about 0.6 deciliters per gram (dL/g), whereupon it is further polymerized in the solid phase to achieve an intrinsic viscosity that better promotes bottle formation. Thereafter, the polyethylene terephthalate may be formed into articles, such as by injection molding preforms, which in turn may be stretch-blow molded into bottles.
Unfortunately, at normal production rates, most polyester resins cannot be efficiently formed into preforms and bottles that are suitable for hot-fill applications. Most high-clarity polyester bottles do not possess the necessary dimensional stability to be hot-filled with product at temperatures between about 180° F. and 205° F., especially between about 195° F. and 205° F. In particular, at such elevated temperature conventional polyester bottles exhibit unacceptable shrinkage and haze. In addition, polyester bottles that package carbonated beverages have been known to suffer stress cracking, which leads to catastrophic failure, or exhibit thermal instability (e.g., thermal creep), which can result in packaging deformations such as (e.g., fill-line drop, base rocking, and label distortion).
Therefore, there is a need for polyethylene terephthalate resin that is suitable for making both high-clarity, hot-fill bottles that can be filled with product at temperatures between about 180° F. and 205° F. and high-strength, high-clarity bottles that possess improved resistance to stress cracking and thermal creep.